In the related art, a transmissive display device is known of which displays an image by illuminating a transmissive display unit such as an LCD (Liquid Crystal Display) panel from a back surface thereof with a surface light source device (backlight). The surface light source devices are mainly classified into direct-type surface light source devices where a light source is arranged just under various optical members, such as an optical sheet, and edge-type surface light source devices where a light source is arranged on a side surface side of optical members. Among the surface light source devices, since the light source is arranged in the side surface side of the optical members such as a light guide plate, the edge-type surface light source device has an advantage of being capable of being thinned further in comparison with the direct-type surface light source device, and thus, in recent years, the edge-type surface light source device has become widely used.
In general, in an edge-type surface light source device, a light source is arranged at a position facing a light incident surface which is a side surface of a light guide plate, light emitted by the light source is incident from the light incident surface on the light guide plate, and the light propagates from the light incident surface toward the surface side facing the light incident surface in a direction (light guide direction) perpendicular to the light incident surface while being repeatedly reflected by a light emitting surface and a back surface facing the light emitting surface. In addition, by changing the light propagation direction by a diffusion pattern, a prism shape, or the like provided on the back surface of the light guide plate, the light emits and propagates gradually from the positions along the light guide direction of the light emitting surface toward the LCD panel side (for example, Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2012-3883